Acetic anhydride has been known as an industrial chemical for many years and large amounts are used in the manufacture of cellulose acetate. It has commonly been produced on an industrial scale by the reaction of ketene and acetic acid. It is also known that acetic anhydride can be produced by the decomposition of ethylidene diacetate, as well as by the oxidation of acetaldehyde, for example. Each of these "classic" processes has well-known drawbacks and disadvantages and the search for an improved process for the production of acetic anhydride has been a continuing one. Proposals for producing anhydrides by the action of carbon monoxide upon various reactants (carbonylation) have been described, for example, in Reppe et al. U.S. Pat. Nos. 2,729,561, 2,730,546 and 2,789,137. However, such prior proposals involving carbonylation reactions have required the use of very high pressures. More recently carbonylation at lower pressures has been proposed, but primarily as a route to the preparation of acetic acid. French Pat. No. 1,573,130, for example, describes the carbonylation of methanol and mixtures of methanol with methyl acetate in the presence of compounds of Group VIII noble metals such as iridium, platinum, palladium, osmium, and ruthenium and in the presence of bromine or iodine under more moderate pressures than those contemplated by Reppe et al. Similarly, South African Pat. No. 68/2174 produces acetic acid from the same reactants using a rhodium component with bromine or iodine. Schultz (U.S. Pat. Nos. 3,689,533 and 3,717,670) has disclosed a vapor phase process for acetic acid production employing various catalysts comprising a rhodium component dispersed on a carrier. None of these later carbonylation disclosures, however, refers to or contemplates the preparation of acetic anhydride or other carboxylic acid anhydrides, and they use the corrosive iodine promoter.
Improved processes for preparing carboxylic acid anhydrides, including acetic anhydride, have been disclosed by Colin Hewlett and Nabil Rizkalla. In these processes Group VIII metals are catalyst components.
In U.S. Pat. Nos. 4,002,677 and 4,002,678, Nagliri et al. use nickel-chromium catalysts at lower pressures than the art prior to produce carboxylic acid anhydrides. However, corrosive iodine promoters are again used.
Rhodium catalyst in combination with iodine promotor effected for acetic anhydride synthesis had also been disclosed in U.S. Pat. Nos. 4,251,458 and 4,284,586.
It is an object of this invention to selectively produce acetic anhydride via carbonylation of the ester derivative of methanol, methyl acetate, using a catalyst system which contains no corrosive iodine promoter required by catalysts of the prior art and which operates at lower pressures.
Further, it is an object of this invention to provide a process for recycling acetic anhydride produced, adding methanol with the recycled product and producing acetic acid.